It's not as simple as charging ad revenue per user.
Only a small subset of users can/would afford to pay $25/yr for Facebook, and these users are arguably the most valuable to advertisers. Therefore by letting them skip ads, FB would lose out on more than $25/yr/user.
It might be profitable at something like $1,000/yr, but who would pay that?
So you just repeated the same argument I replied to, without adding anything new. It absolutely does not follow to me that the people who could and would pay for an ad-free experience are the most valuable to advertisers. As I pointed out, I am one of those people, and I expect I am worth nearly nothing to advertisers. Sure, there's certainly a subset of the would/could crowd who click on ads like crazy and buy a bunch of crap, but is that in the majority? I wouldn't think so.
And besides, there's always a number that works out to a break-even point. Maybe it's above $25/yr, but suggesting it's as high as $1000/yr is absurd. Even if it's $100/yr, I'd still bite (not for FB, but for a site I'd actually get use out of to the degree that many people do with FB). Sure, that further reduces the pool of people who could/would pay, but it'd still be an available option for some people.
Only a small subset of users can/would afford to pay $25/yr for Facebook, and these users are arguably the most valuable to advertisers. Therefore by letting them skip ads, FB would lose out on more than $25/yr/user.
It might be profitable at something like $1,000/yr, but who would pay that?